In paper manufacturing trimmed strips are cut from a wet web, and additionally wet broke material will be generated at the wire and the press section in connection with a web break. Usually the wet broke material is collected, disintegrated and diluted in a pulper comprising a container with a powerful mixing feature and thereafter it is returned to the process.
For the disintegration to function properly the pulper must comprise a sufficient amount of mass or stock which is maintained under a powerful mixing by the disintegration means of the pulper. During regular processing the amount of stock brought to the pulper corresponds to a minor part of a paper machine's production. At web breaks, changes of quality and machine stops the amount of stock may temporarily rise to an amount corresponding to the total production capacity of the machine.
In order to achieve a satisfactory mixing and disintegration effect the stock in the pulper must be kept highly diluted and a sufficiently large volume of mass must be kept therein. This leads to a high power demand for the disintegration and mixing.
At a web break the water consumption rises greatly and for this purpose a water reserve must be arranged corresponding to the demand at a typical web break. After the disintegration the wet broke material usually is de-watered in a separate de-watering device for better correspondence with the production consistency and for retrieving the water therefrom.
At production changes, particularly when the color is changed, there exists a large amount of wet broke material from the earlier paper run, which cannot be used for the next paper run. The wet broke material in circulation in the system also slows down progress in attaining the new paper quality, resulting in production losses and change-over broke masses.
It is also important that the pulper be easily and completely emptied of its contents, which is not the case for typical pulpers.
Many paper machines have a low and narrow wire section which does not provide space for constructing an appropriate pulper. In this case, vaulting and broke stocking problems will arise, and the pulper itself can be difficult to maintain and service.
Corresponding problems occur in other productions in which a mass or different streams of components are to be processed in order to achieve a homogeneous mass.
Usually the wet broke mass is diluted to a 2 to 3 percent consistency in a couch pit or wire pulper. Depending upon the structure of the paper machine, a similar pulper may also be arranged in connection with the press section. Depending upon the size of the machine, the wire pulper is typically between five and fifty cubic meters, and it comprises a powerful mixer which breaks up fiber bundles and pieces of paper. In order to function effectively, the pulper requires good mixing which, in turn, requires a sufficient dilution of the mass. Since the pulper works under a variety of conditions, the dilution usually is over-dimensioned according to the most critical situation.